Re: [PATCH 09/39] drivers/usb/mon: update to use mmap_prepare + map kernel pages

On Tue, Sep 08, 2026 at 09:01:13PM +0100, Lorenzo Stoakes (ARM) wrote:

Replace the deprecated .mmap hook with its replacement .mmap_prepare. As
part of this change, additionally take the approach of mapping pages upon
mmap rather than providing a fault handler.

The page span cannot be mutated when an mmap mapping is in place, so this
is safe to do in advance (the MON_IOCT_RING_SIZE ioctl operation exits
-EBUSY if it's attempted, gated by the rp->mmap_active reference count).

Utilise the newly introduced mmap_action_map_discontig_kernel_pages() to do
this, which allows for iteration over pages in mon_bin_discontig_get().

mon_bin_discontig_init() increments the rp->mmap_active reference count to
stabilise page spans. Should an error arise the core unmaps the VMA and
mon_bin_vma_close() drops the reference again.

The vm_ops->close hook implemented in mon_bin_vma_close() will ensure
correct reference count arithmetic upon unmap (with mon_bin_vma_open()
accounting for splitting).

The existing semantics are all retained, including not mapping past the
range of available pages, with a SIGBUS being raised in a userland process
that attempts to access past this point.

Ultimately insert_page() is invoked to insert each page, which increments
the reference count on each mapped page. This mimics what was being done
previously, only we pre-map the entire range rather than doing so on
demand.

The existing fault handler did nothing that required demand paging, and was
presumably implemented this way due for historic reasons.

One behavioural difference: pages are no longer faulted in on demand, so a
page discarded with MADV_DONTNEED is not repopulated and a subsequent
access raises SIGBUS, as with other pre-populated kernel mappings.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>